The realisation of electric motors is known in the art, where a stator has a cylindrical cavity and a cylindrical rotor is arranged inside the stator cavity, the rotor being configured to rotate under the action of a magnetic field generated by the stator.
In some applications, the motor is immersed in a fluid, which flows also inside the gap separating the rotor from the stator. Due to the rotation of the rotor respect to the stator, the fluid flows in the gap in a turbulent way, generating friction losses and reducing the motor mechanical efficiency.
Moreover, the fluid in the gap gets overheated by friction, and raises the temperature of the whole motor, generating obvious problems. Friction power losses particularly increase if the fluid in the gap has a high viscosity and the motor rotates at a high speed.
This is particularly the case of apparatuses where electric motors are used to drive directly compressors and are arranged in a same housing with the compressor, for example in methane pipeline applications. In fact, such motors must rotate at a high speed, for example 10000 rpm, and the process gas flowing in the motor can be at high pressure, for example 80 bar to 100 bar, and consequently the gas has a high viscosity and generates high friction losses and a heat amount much higher than the common electromagnetically generated heat. Another application with similar problems is in refrigeration apparatuses, where the processed fluid, for example R134A, has a high viscosity at a pressure of 3 bar.
Because of this and other problems, the electric motors for driving compressors are often arranged in housings separated from the compressor housing, in order to avoid the fluid from flowing in the machine gap. However, this kind of solution does not ensure the fluid sealing of the compressor housing, which must have an opening to receive the drive shaft. Moreover, the sealing systems for the shaft brings to additional mechanical losses and to the consumption of the apparatus.
Otherwise, the motor flooded by the compressor process fluid must be provided with a strong cooling system, in particular in order to cool the stator of the motor, which is its most temperature sensitive part.
Document WO 2011099603 discloses a motor apparatus with a pump and an electric motor, where the liquid processed by the pump circulates also around the rotor. A can, made of materials with high resistance to pressure, temperature and erosion, completely enclose the stator, preserving it from contact with the turbulent and hot fluid.
In this document, a cooling system is provided in order to cool the stator chamber, the cooling system having a dedicated compressor and a dedicated fluid, which is separated from the process fluid and dissipates its heat in an external heat exchanger. Since the pressures of two distinct fluids apply on the surfaces of the can, a pressure balancing system must be provided, otherwise the can could be deformed or even broken.
However, such a balancing system requires high performances and high costs, since it may result not fast enough in compensating the pressure unbalance during a transient of the pump work. Moreover, any deformation of the can may result in problems for the sealing of the stator, eventually bringing to the mixing of the pump liquid and the cooling gas and therefore to the damaging of the compressor and the pump.
Other documents such as WO 2010014647, JP 2001231213 and WO 2013131820 disclose the possibility of enclosing a stator of an electrical machine in a can for its insulation from a turbulent fluid flowing around the rotor. However, all these documents face the problem of the pressure applied to the can by reinforcing the can or by trying to regulate an inner pressure of the can.
Object of the present invention is to provide an electric motor adapted to work in a fluid obviating the aforementioned prior art drawbacks, and an in-line motor-compressor apparatus with a simplified and reliable system for maintaining the stator temperature at acceptable values also when subject to high pressures or fast pressure variations.